Field of the Invention
An adhesive composition containing a polyorgano-silsesquioxane powder ((R—SiO3/2)n) and a silver powder. The adhesives are useful as a bonding member to bond components of electronic, optoelectronic, and semiconductor devices. The adhesives exhibit high performance and desirable thermal conductivity.
Background Information
Advances in the electronic, optoelectronic, and semiconductor industries have driven the need for high performance adhesives. In particular, high power devices require low-stress, high thermal conductivity, thermally stable, and moisture resistant adhesives for the manufacture of high reliability devices. Specifically, high power semiconductor devices require high thermal conductivity adhesives to efficiently drain heat from the semiconductor components (chips or dies) of the device, so that long-term performance (i.e., the functional properties) of the device does not degrade.
The manufacture of semiconductor devices typically involves attaching a semiconductor die (such as made from silicon, silicon carbide, silicon nitride, aluminum nitride or gallium nitride) to a substrate (such as a ceramic, copper, or a copper alloy), or a circuit board, using a thermally conductive adhesive. FIG. 1 illustrates a semiconductor device, wherein the underside of the die (or chip) is optionally metallized with gold or silver, and the adhesive is used as a bonding member to attach the die (or chip) to a substrate that is metallized with copper, which is optionally metallized with gold or silver. The process of attaching the semiconductor die to a substrate using an adhesive as the bonding member will be referred to hereinafter as “die attach”. The adhesive layer between the die and the substrate is often referred to as the “bondline.”
Adhesives that have heretofore been used for a surface mount, or die attach, generally fall into categories based on the paste formulation, the functional performance, and the use temperature. In general, silver die attach adhesives can be categorized into three types. One being a resin-type adhesive, another being a sintering-type adhesive, and the third being an inorganic silver-glass sintering-type adhesive.
A typical resin-type (organic/inorganic) die attach adhesive may contain the following:
(a) silver powders (with a diameter of about 0.5 to 50 micrometers, which are of a spherical shape, an irregular shape, or a flake shape),
(b) a thermosetting resin (e.g., an epoxy resin, an acrylic resin, a cyanate resin, and mixtures thereof),
(c) a thermoset hardener,
(d) a solvent, and
(e) optionally one or more additives (e.g., a coupling agent, a dispersing agent, or a surfactant).
A typical sintering-type die attach adhesive (which can be inorganic or organic/inorganic) may contain the following:
(a) silver nano-particles (having a diameter of 5 to 500 nanometers; which can be sintered below 350° C.),
(b) a solvent,
(c) optionally silver powders (having a diameter of about 0.2 to 20 micrometers;
(d) optionally a resin (a thermosetting resin and/or a thermoplastic resin),
(e) optionally a sintering aid (a specific dispersant may behave as a sintering aid), and
(f) optionally one or more additives (e.g., a coupling agent, a dispersant, or a surfactant).
A typical inorganic silver-glass die attach adhesive may contain the following:
(a) silver particles (having a diameter of about 0.2 to 20 micrometers),
(b) a glass frit (low melting temperature glass powder),
(c) optionally a filler (e.g., an inorganic metal oxide power),
(d) a solvent, and
(e) optionally one or more additives (e.g., a dispersant, a surfactant, or a rheological agent).
In the case of organic/inorganic adhesives, there are functional limitations due to the chemical make-up of the adhesive. For example, the organic components (e.g., epoxy and thermoplastic materials) have temperature limitations due to the decomposition of the organic material. Typical die attach adhesives that contain epoxy and/or thermoplastic components have been reported to have a maximum use temperatures of 200° C. to 225° C. These adhesives exhibit degradation in properties (such as a high temperature die shear strength), when manufactured devices are exposed to additional processing steps such as a high temperature solder (e.g., Pb-free SAC305, or gold-tin solder) reflow temperatures of 250° C. to 320° C. These adhesives also show degradation in performance when exposed to high use temperatures (e.g., >200° C.) for a long-term use. Degradation of the adhesive, especially formation of cracks or degradation of functional properties (e.g., thermal conductivity), results in decreased performance of the manufactured semiconductor device.
In addition to temperature limitations, organic/inorganic adhesives often have a trade-off between achieving a low modulus adhesive (low-stress) and resistance to moisture attack, where the organic component of the paste component typically is most susceptible to moisture attack. Also, low modulus adhesives of this type tend to have a significantly lower thermal conductivity due to the high polymer resin content. All of these factors lead to problems with respect to the long-term reliability for high temperature (high power), high performance devices.
There are times, however, when organic/inorganic adhesives are desirable. For example, when die attach applications use a bare (no metallization) die and/or a bare substrate, an organic component is often used in the adhesive formulation to achieve good adhesion. For some applications, such as those that do not require high temperature operation, use of bare die attach components can result in cost savings with respect to the manufacture of the device. In addition, if the silver powder component of the paste formulation does not sinter below the typical processing temperatures (i.e., ≤200° C.) of the adhesive, organic components are added to the paste formulation to meet the functional requirements of the adhesive.
Adhesives that are primarily inorganic in composition, such as the sintering-type of adhesives, display excellent properties regarding thermal conductivity and thermal stability; however, storage modulus values are typically high. High storage modulus adhesives are typically not suitable for large die applications, or applications where there is a large thermal expansion mismatch between the die and substrate, due to a high stress at the adhesive bondline. Organic components can be added to the sintering-type adhesives to modify the functional properties, such as decreasing the storage modulus for lower stress bondlines.
In the case of thermally conductive silver-glass composites, U.S. provisional patent application Ser. No. 61/928,533 filed on Jan. 17, 2014, entitled “CONDUCTIVE PASTE AND METHOD FOR PRODUCING A SEMICONDUCTOR DEVICE USING THE SAME”, inventors Raymond Dietz et al. (WO 2015/108205) was directed to a thermally conductive metal and low melting temperature glass frit (powder) blended together with a solvent system (a vehicle) to make a die attach adhesive for high temperature applications (e.g., up to a 300° C. continuous use temperature). This type of adhesive formulation is robust at elevated temperature use, and performs well with respect to reliability testing of a silicon carbide die bonded to a silver metallized alumina substrate. The silver-glass adhesives also show excellent performance when used to bond a bare die and a bare substrate together during the manufacture of the die attach device.
While silver-glass adhesives show superior performance up to a 300° C. use temperature, the processing temperature for the adhesive is about 370° C., which is too high for some temperature sensitive electronic devices or packages. In addition, the storage modulus for silver-glass composite adhesives is quite high, due to the rigid, brittle nature of glass and glass ceramic materials. For this reason, such adhesive is not suitable for large die applications or applications where there is a large thermal expansion mismatch between the die and the substrate, due to a high stress at the adhesive bondline.
Metal and metal-alloy solders are another option for surface mount (die attach) solders. Lead-based solders are being replaced by lead-free solder options, such as SAC305 solder, for a lead-free assembly of electronic components and devices. SAC305 solder is commonly used in industry as a lead-free solder replacement for lead-based solders; and often uses a flux for the soldering process, which can result in a post-soldering residue on the components. In addition, the SAC305 solder requires processing temperatures in the range of 250 to 270° C., with common wave soldering processes requiring about 265° C. Reliability testing with SAC305 solder has shown problems with the formation of intermetallic phase(s) at interfaces, which contribute to problems with reliability testing of soldered components and devices.
High temperature gold-based solders, such as gold, gold-tin, gold-silicon, and gold-germanium, are used for various die attach applications that require high temperature performance. Stamped solder preforms are often used in surface mount applications to eliminate the need for flux during the soldering process; however, a low oxygen atmosphere (such as nitrogen, or a mixture of nitrogen and hydrogen) is required for the soldering process. Gold-based solders require processing temperatures greater than 280° C., and the thermal profile for a gold-tin solder, which is used for die attach applications, can reach 320° C. for short times, with an addition of external pressure applied on the die attach part during processing to achieve satisfactory die attach bondline and interfaces. Gold-tin solders exhibit a modest thermal conductivity of about 57 W/mK.
Polymethylsilsesquioxane powders, (CH3—SiO3/2)n, are commonly used in the cosmetic industry, and are commercially available in a wide range of particle sizes of interest for use in die attach adhesives (e.g. 0.8 to 20 micrometers). FIG. 2 is a schematic of a polymethylsilsesquioxane particle, wherein siloxane bonds have formed to create a 3-dimensional network; the methyl groups are not cross-linked, which result in a low modulus material. Polymethylsilsesquioxane fine powders are commercially available, with products from companies such as Grant Industries, Inc., Elmwood Park, N.J. and ABC NANOTECH, Daejeon, Korea (see the scanning electron images of FIG. 3). While these fine powders differ in particle size, the physical properties are similar, as shown in Table 1.
TABLE 1Appearance and properties of polymethylsilsesquioxane powderAppearancewhite powderRefractive Index1.43Density (g/cc)1.3Melting Point>999° C.
The polymethylsilsesquioxane powder data sheets from ABC NANOTECH also show data which indicate exceptional thermal stability of the fine powders. This property was confirmed as shown in FIG. 4, wherein a thermal analysis of the powders (using a differential scanning calorimetry) shows decomposition of the methyl groups (exothermic peak) at temperatures >350° C.
In addition, polymethylsilsesquioxane materials do not melt or flow (there is no glass transition temperature (Tg)) in the temperature range of interest (−55° C. to 350° C.) for many die attach applications. Therefore, “re-melting” of the material at elevated temperatures or during thermal cycling of the device is not a concern. Also, since there is no Tg in the temperature range of interest, there are no rapid changes in properties, such as storage modulus or thermal expansion, that are typically observed for many epoxy and polymer (e.g., thermoplastic) materials.
U.S. Pat. No. 8,835,574 to Nguyen et al. is directed to adhesive compositions for use in die attach applications. Column 18, line 25 of U.S. Pat. No. 8,835,574 discloses that polymethylsilsesquioxane powder can be used as a filler in the Nguyen et al. adhesive composition.
JP S62-128162 concerns an epoxy resin composition for encapsulating a semiconductor. The composition contains the following:
(a) an epoxy resin,
(b) a hardener,
(c) a silicone powder described as (RSiO3/2)n, and
(d) a silicone oil
In JP S62-128162, it is stated that the composition serves to reduce the stress of a semiconductor device.
JP 2003-347322 is directed to a die-attach paste which contains a thermosetting resin and a polyorganosilsesquioxane powder. This is an insulating paste and does not contain silver powder.
JP 2009-013294 describes the use of silver flakes (“A1”) or spherical polyorganosilsesquioxane powder (“A2”) as a filler for a die attach adhesive (see paragraphs[0194] et seq.). However, silver flakes and polyorganosilsesquioxane powder are not simultaneously used in the same paste (see Table 1).
JP 2010-003848 discloses die attach adhesives for an LED containing polyorganosilsesquioxane powders.
U.S. Pat. No. 5,415,912 concerns a pressure sensitive adhesive (an adhesive tape) that contains polyorganosilsesquioxane fine particles.
U.S. Pat. No. 5,827,921 relates to a silicone-based material containing a component comprising colloidal silica or polyorganosilsesquioxane material.
US 2012/0114927 and US 2013/0266796 deal with Nano-Ag adhesives and thermal bonding adhesives, respectively.